Optical device, laser beam source, laser apparatus and method of producing optical device

ABSTRACT

After forming domain inverted layers  3  in an LiTaO 3  substrate  1 , an optical waveguide is formed. By performing low-temperature annealing for the optical wavelength conversion element thus formed, a stable proton exchange layer  8  is formed, where an increase in refractive index generated during high-temperature annealing is lowered, thereby providing a stable optical wavelength conversion element. Thus, the phase-matched wavelength becomes constant, and variation in harmonic wave output is eliminated. Consequently, with respect to an optical wavelength conversion element utilizing a nonlinear optical effect, a highly reliable element is provided.

This application is a divisional of U.S. patent application Ser. No.09/922,978 filed Aug. 6, 2001 which is a divisional of U.S. patentapplication Ser. No. 08/973,380, now U.S. Pat. No. 6,333,943, The entiredisclosures of U.S. patent application Ser. No. 09/922,978 filed Aug. 6,2001 and U.S. Pat. No. 6,333,943 are expressly incorporated by referenceherein.

TECHNICAL FIELD

The present invention relates to an optical element such as an opticalwavelength conversion element, a laser light source and a laser devicesuitable for use in the field of optical information processing oroptical measuring control where coherent light is used, and also relatesto a method for producing an optical element.

BACKGROUND ART

Referring to FIG. 1, a conventional laser light source using an opticalwavelength conversion element will be described. The laser light sourceis basically composed of a semiconductor laser 20, a solid state lasercrystal 21 and an optical wavelength conversion element 25 made ofKNbO₃, which is a non-linear optical crystal.

As shown in FIG. 1, pumped light P1 a emitted from the semiconductorlaser 20, which oscillates at 807 nm, is collected by a lens 30 so as toexcite YAG as a solid state laser crystal 21. A total reflection mirror22 is formed on an incident surface of the solid state laser crystal 21.The total reflection mirror reflects 99% of light having a wavelength of947 nm but transmits light in the 800 nm wavelength band. Although thepumped light P1 a is thus efficiently introduced into the solid statelaser crystal 21, the light with a wavelength of 947 nm, which isgenerated by the solid state laser crystal 21, is reflected to theoptical wavelength conversion element 25 side without being emitted tothe semiconductor laser 20 side. Moreover, a mirror 23, which reflects99% of light having a wavelength of 947 nm but transmits light in the400 nm wavelength band, is provided on the output side of the opticalwavelength conversion element 25. These mirrors 22 and 23 form aresonator (cavity) for light having a wavelength of 947 nm, capable ofgenerating oscillation at 947 nm as a fundamental wave P1.

The optical wavelength conversion element 25 is inserted in the cavitydefined by the mirrors 22 and 23, whereby a harmonic wave P2 isgenerated. The power of the fundamental wave P1 within the cavityreaches to 1 W or higher. Therefore, the conversion from the fundamentalwave P1 to the harmonic wave P2 is increased, whereby a harmonic wavehaving a high power can be obtained. A harmonic wave of 1 mW can beobtained by using a semiconductor laser having an output of 500 mW.

Next, referring to FIG. 2, a conventional optical wavelength conversionelement having an optical waveguide will be described. The illustratedoptical wavelength conversion element, when a fundamental wave having awavelength of 840 nm is incident thereupon, generates a secondaryharmonic wave (wavelength: 420 nm) corresponding to the fundamentalwave. Such an optical wavelength conversion element is disclosed in K.Mizuuchi, K. Yamamoto and T. Taniuchi, Applied Physics Letters, Vol 58,p. 2732, June 1991.

As shown in FIG. 2, in this optical wavelength conversion element, anoptical waveguide 2 is formed in an LiTaO₃ substrate 1, with layerswhose polarization is inverted (domain inverted layers) 3 beingperiodically arranged along the optical waveguide 2. Portions of theLiTaO₃ substrate 1 where the domain inverted layer 3 is not formed willserve as a domain non-inverted layer 4.

When the fundamental wave P1 is incident upon one end (an incidentsurface 10) of the optical waveguide 2, the harmonic wave P2 is createdin the optical wavelength conversion element and is output from theother end of the optical waveguide 2. At this point, light propagatingthrough the optical waveguide 2 is influenced by a periodic structureformed by the domain inverted layers 3 and the domain non-inverted layer4, whereby propagation constant mismatching between the generatedharmonic wave P2 and the fundamental wave P1 is compensated by theperiodic structure of the domain inverted layers 3 and the domainnon-inverted layer 4. As a result, the optical wavelength conversionelement is able to output the harmonic wave P2 with a high efficiency.

Such an optical wavelength conversion element includes, as a basiccomponent, the optical waveguide 2 produced by a proton exchange method.

Hereinafter, referring to FIG. 3, a method for producing such an opticalwavelength conversion element will be described.

First, at step S10 in FIG. 3, a domain inverted layer formation step isperformed.

More particularly, a Ta film is first deposited so as to cover theprincipal surface of the LiTaO₃ substrate 1, after which ordinaryphotolithography and dry etching techniques are used to pattern the Tafilm into a striped pattern, thereby forming the Ta mask.

Next, a proton exchange process is performed at 260° C. for 20 minutesfor the LiTaO₃ substrate 1 whose principal surface is covered by the Tamask. Thus, 0.5 μm thick proton exchange layers are formed in portionsof the LiTaO₃ substrate 1 which are not covered by the Ta mask. Then,the Ta mask is removed by etching for 2 minutes using a mixturecontaining HF:HNF₃ at 1:1.

Next, a domain inverted layer is formed within each of the protonexchange layers by performing a heat treatment at 550° C. for 1 minute.In the heat treatment, the temperature rise rate is 50° C./sec and thecooling rate is 10° C./sec. In portions of the LiTaO₃ substrate 1 wherethe proton exchange has been performed, the amount of Li is reduced ascompared to that in other portions thereof where the proton exchange hasnot been performed. Therefore, the Curie temperature of the protonexchange layer decreases, whereby the domain inverted layer can beformed partially in the proton exchange layer at a temperature of 550°C. This heat treatment allows for formation of the proton exchange layerhaving a pattern upon which the pattern of the Ta mask is reflected.

Next, at step 2 in FIG. 3, an optical waveguide formation step isperformed.

More particularly, step 2 is generally divided into step S21, step S22and step S23. The mask pattern is formed at step S21; the protonexchange process is performed at step S22; and high-temperatureannealing is performed at step S23.

These steps will be described below.

At step S21, the Ta mask used for forming the optical waveguide isformed. The Ta mask is obtained by forming slit-shaped openings (width:4 μm, length: 12 mm) in a Ta film. At step S22, a high refractive indexlayer (thickness: 0.5 μm) linearly extending in one direction is formedin the LiTaO₃ substrate 1 by performing a proton exchange process at260° C. for 16 minutes for the LiTaO₃ substrate 1 which is covered bythe Ta mask. The high refractive index layer will eventually function asan optical waveguide. However, the non-linearity of the portions wherethe proton exchange has been performed (the high refractive indexlayers), as thus formed, is deteriorated. In order to restore thenon-linearity, annealing is performed at 420° C. for 1 minute at stepS22 after removing the Ta mask. This annealing expands the highrefractive index layer in the vertical direction and in the lateraldirection, thereby diffusing Li into the high refractive index layers.By reducing the proton exchange concentration in the high refractiveindex layers in this way, it is possible to restore the non-linearity.As a result, the refractive index of the regions located directly underthe slits of the Ta mask (the high refractive index layers) is increasedby about 0.03 from the refractive index in other regions, whereby thehigh refractive index layers function as an optical waveguide.

Next, a protective film formation step (step S30), an end face polishingstep (step S40), and an AR coating step (step S50) are performed,thereby completing an optical wavelength conversion element.

By setting the arrangement pitch of the domain inverted layersperiodically arranged along the waveguide to 10.8 μm, it is possible toform a third-order pseudo phase-matched structure.

With the above-described optical wavelength conversion element, when thelength of the optical waveguide 2 is set to 9 mm, the harmonic wave P2having a power of 0.13 mW can be obtained for the fundamental wave P1(power: 27 mW) having a wavelength of 840 nm (conversion efficiency:0.5%).

For forming a first-order pseudo phase-matched structure, thearrangement pitch of the domain inverted layers can be set to 3.6 μm. Inthis case, the harmonic wave P2 of 0.3 mW can be obtained for thefundamental wave P1 of 27 mW (conversion efficiency: 1%). The inventorsof the present invention have experimentally produced a laser lightsource which outputs blue laser light by combining such an opticalwavelength conversion element with a semiconductor laser.

Such an optical wavelength conversion element has a problem that thephase-matched wavelength thereof varies with the passage of time,whereby a harmonic wave cannot be obtained. When the wavelength of thefundamental wave emitted from a semiconductor laser is kept constant,but the phase-matched wavelength of the optical wavelength conversionelement is shifted, the harmonic wave output will gradually decrease,and it will eventually becomes zero.

The object of the present invention is to stabilize a laser lightsource, to increase the output thereof, and to reduce the size andweight of a laser device or an optical disk apparatus by incorporating ahigh output laser light source into these devices/apparatuses.

DISCLOSURE OF THE INVENTION

A method for producing an optical element of the present inventionincludes: a step of forming a proton exchange layer in anLiNb_(x)Ta_(1-x)O₃ (0≦X≦1) substrate; and an annealing step ofperforming a heat treatment for the substrate at a temperature of 120°C. or lower for 1 hour or more.

Preferably, the annealing step is performed at a temperature equal to orhigher than 50° C. but lower than or equal to 90° C.

The annealing step may include a step of gradually lowering thetemperature.

In one embodiment, the step of forming the proton exchange layerincludes: a step of performing a proton exchange process for thesubstrate; and a step of performing a heat treatment for the substrateat a temperature of 150° C. or higher.

In one embodiment, the step of forming the proton exchange layerincludes: a step of forming a plurality of periodically-arranged domaininverted layers in the substrate; and a step of forming an opticalwaveguide on a surface of the substrate.

Another method for producing an optical element of the present inventionincludes: a step of performing a proton exchange process for anLiNb_(x)Ta_(1-x)O₃ (0≦X≦1) substrate; and an annealing step ofperforming a plurality of heat treatments including at least first andsecond heat treatments for the substrate. The temperature of the secondannealing is lower than the temperature of the first annealing by 200°C. or more.

Preferably, the second annealing is performed at a temperature equal toor higher than 50° C. but lower than or equal to 90° C.

An optical element of the present invention includes anLiNb_(x)Ta_(1-x)O₃ (0≦X≦1) substrate and a proton exchange layer formedin the substrate. The optical element is formed of a stable protonexchange layer such that a refractive index of the proton exchange layerdoes not vary with time during operation.

In one embodiment, at least a portion of the proton exchange layer formsan optical waveguide.

A light source of the present invention includes: a semiconductor laser;and an optical wavelength conversion element for receiving laser lightemitted from the semiconductor laser so as to convert the laser light toa harmonic wave. The optical wavelength conversion element includes: anoptical waveguide for guiding the laser light; and domain invertedstructures periodically arranged along the optical waveguide, theoptical waveguide and the domain inverted structures being formed of astable proton exchange layer whose refractive index does not vary withtime during operation.

Another laser light source of the present invention includes: asemiconductor laser for emitting a fundamental wave; a single mode fiberfor conveying the fundamental wave; and an optical wavelength conversionelement for receiving the fundamental wave emitted from the fiber so asto generate a harmonic wave, the optical wavelength conversion elementhaving periodic domain inverted structures.

In one embodiment, the optical wavelength conversion element has amodulation function.

Preferably, the optical wavelength conversion element is formed in anLiNb_(x)Ta_(1-x)O₃ (0≦X≦1) substrate.

Still another laser light source of the present invention includes: asemiconductor laser for emitting a pumped light; a fiber for conveyingthe pumped light; a solid state laser crystal for receiving the pumpedlight emitted from the fiber so as to generate a fundamental wave; andan optical wavelength conversion element for receiving the fundamentalwave so as to generate a harmonic wave, the optical wavelengthconversion element having periodic domain inverted structures.

Preferably, the optical wavelength conversion element has a modulationfunction.

Preferably, the optical wavelength conversion element is formed in anLiNb_(x)Ta_(1-x)O₃ (0≦X≦1) substrate.

In one embodiment, the solid state laser crystal and the opticalwavelength conversion element are integrated together.

Still another laser light source of the present invention includes: asemiconductor laser for emitting a pumped light; a solid state lasercrystal for receiving the pumped light so as to generate a fundamentalwave; a single mode fiber for conveying the fundamental wave; and anoptical wavelength conversion element for receiving the fundamental wavefrom the fiber so as to generate a harmonic wave, the optical wavelengthconversion element having periodic domain inverted structures.

Preferably, the optical wavelength conversion element has a modulationfunction.

Still another laser light source of the present invention includes: adistributed feedback type semiconductor laser for emitting laser light;a semiconductor laser amplifier for amplifying the laser light; and anoptical wavelength conversion element for receiving the amplified laserlight so as to generate a harmonic wave, the optical wavelengthconversion element having periodic domain inverted structures.

Preferably, the optical wavelength conversion element has a modulationfunction.

Preferably, the optical wavelength conversion element is formed in anLiNb_(x)Ta_(1-x)O₃ (0≦X≦1) substrate.

In one embodiment, the semiconductor laser is wavelength-locked.

Still another laser light source of the present invention includes: asemiconductor laser for emitting laser light; and an optical wavelengthconversion element in which periodic domain inverted structures and anoptical waveguide are formed. The width and the thickness of the opticalwaveguide are each 40 μm or greater.

In one embodiment, the optical wavelength conversion element has amodulation function.

The optical wavelength conversion element is formed in anLiNb_(x)Ta_(1-x)O₃ (0≦X≦1) substrate.

In one embodiment, the optical waveguide is of a graded type.

A laser device of the present invention includes: a laser light sourcehaving a semiconductor laser for radiating laser light and an opticalwavelength conversion element for generating a harmonic wave based onthe laser light; a modulator for modulating an output intensity of theharmonic wave; and a deflector for changing a direction of the harmonicwave emitted from the laser light source. Periodic domain invertedstructures are formed in the optical wavelength conversion element.

In one embodiment, a harmonic wave is superimposed over thesemiconductor laser during operation.

In one embodiment, the laser light source includes a single mode fiberfor conveying laser light from the semiconductor laser to the opticalwavelength conversion element.

In one embodiment, the laser light source includes: a fiber forconveying laser light from the semiconductor laser; and a solid statelaser crystal for receiving laser light emitted from the fiber so as togenerate a fundamental wave.

In one embodiment, the semiconductor laser device is a distributedfeedback type semiconductor laser; and the laser light source furthercomprises a semiconductor laser amplifier for amplifying the laser lightfrom the distributed feedback type semiconductor laser.

In one embodiment, an optical waveguide is formed in the opticalwavelength conversion element; and the width and the thickness of theoptical waveguide are each 40 μm or greater.

Another laser device of the present invention includes: a laser lightsource for radiating modulated ultraviolet laser light; and a deflectorfor changing a direction of the ultraviolet laser light. The deflectorirradiates a screen with the ultraviolet laser light so as to generatered, green or blue light from a fluorescent substance being applied onthe screen.

In one embodiment, the laser light source includes: a semiconductorlaser; an optical wavelength conversion element for generating aharmonic wave; and a single mode fiber for conveying laser light fromthe semiconductor laser to the optical wavelength conversion element.

In one embodiment, the laser light source includes: a semiconductorlaser; a fiber for conveying laser light from the semiconductor laser; asolid state laser crystal for receiving laser light emitted from thefiber so as to generate a fundamental wave; and an optical wavelengthconversion element for generating a harmonic wave from the fundamentalwave.

In one embodiment, the laser light source further includes: asemiconductor laser; and a semiconductor laser amplifier for amplifyinglaser light from a distributed feedback type semiconductor laser.

In one embodiment, the laser light source includes: a semiconductorlaser for emitting laser light; and an optical wavelength conversionelement in which an optical waveguide for guiding the laser light andperiodic domain inverted structures are formed. The width and thethickness of the optical waveguide are each 40 μm or greater.

Still another laser device of the present invention includes: threelaser light sources for generating red, green and blue laser lightbeams; a modulator for changing an intensity of each of the laser lightbeams; and a deflector for changing a direction of each of the laserlight beams. The laser light source is formed of a semiconductor laser.

In one embodiment, a harmonic wave is superimposed over thesemiconductor laser during operation.

In one embodiment, the laser light source includes: a semiconductorlaser; an optical wavelength conversion element for generating aharmonic wave; and a single mode fiber for conveying laser light fromthe semiconductor laser to the optical wavelength conversion element.

In one embodiment, the laser light source includes: a semiconductorlaser; a fiber for conveying laser light from the semiconductor laser; asolid state laser crystal for receiving laser light emitted from thefiber so as to generate a fundamental wave; and an optical wavelengthconversion element for generating a harmonic wave from the fundamentalwave.

In one embodiment, the laser light source further includes: asemiconductor laser; and a semiconductor laser amplifier for amplifyinglaser light from a distributed feedback type semiconductor laser.

In one embodiment, the laser light source includes: a semiconductorlaser for emitting laser light; and an optical wavelength conversionelement in which an optical waveguide for guiding the laser light andperiodic domain inverted structures are formed. The width and thethickness of the optical waveguide are each 40 μm or greater.

Still another laser device of the present invention includes: at leastone laser light source including a semiconductor laser; asub-semiconductor laser; a modulator for changing an intensity of lightfrom the laser light source; a screen; and a deflector for changing adirection of light from the laser light source so as to scan the screenwith the light. Light emitted from the sub-semiconductor laser scans aperipheral portion of the screen; and radiation of laser light from thelaser light source is terminated when an optical path of the lightemitted from the sub-semiconductor laser is blocked.

In one embodiment, the laser light source includes: an opticalwavelength conversion element for generating a harmonic wave; and asingle mode fiber for conveying laser light from the semiconductor laserto the optical wavelength conversion element.

In one embodiment, the laser light source includes: the semiconductorlaser; a fiber for conveying laser light from the semiconductor laser; asolid state laser crystal for receiving laser light emitted from thefiber so as to generate a fundamental wave; and an optical wavelengthconversion element for generating a harmonic wave from the fundamentalwave.

In one embodiment, the semiconductor laser is a distributed feedbacktype semiconductor laser; and the laser light source further includes asemiconductor laser amplifier for amplifying laser light from thedistributed feedback type semiconductor laser.

In one embodiment, the laser light source includes an optical wavelengthconversion element in which an optical waveguide for guiding laser lightfrom the semiconductor laser and periodic domain inverted structures areformed. The width and the thickness of the optical waveguide are each 40μm or greater.

A laser device of the present invention includes: at least one laserlight source including a semiconductor laser; a deflector for changing adirection of laser light radiated from the laser light source so as toscan the screen with the laser light. The device further comprises twoor more detectors for generating a signal when receiving a portion ofthe laser; and generation of laser light from the laser light source isterminated when the detector does not generate a signal for a certainperiod of time while the deflector scans the screen with the laserlight.

In one embodiment, the laser light source includes: an opticalwavelength conversion element for generating a harmonic wave; and asingle mode fiber for conveying laser light from the semiconductor laserto the optical wavelength conversion element.

In one embodiment, the laser light source includes: the semiconductorlaser; a fiber for conveying laser light from the semiconductor laser; asolid state laser crystal for receiving laser light emitted from thefiber so as to generate a fundamental wave; and an optical wavelengthconversion element for generating a harmonic wave from the fundamentalwave.

In one embodiment, the semiconductor laser is a distributed feedbacktype semiconductor laser; and the laser light source further includes asemiconductor laser amplifier for amplifying laser light from thedistributed feedback type semiconductor laser.

In one embodiment, the laser light source includes an optical wavelengthconversion element in which an optical waveguide for guiding laser lightfrom the semiconductor laser and periodic domain inverted structures areformed. The width and the thickness of the optical waveguide are each 40μm or greater.

Still another laser device of the present invention includes: at leastone laser light source including a semiconductor laser; a modulator forchanging an intensity of each laser light; and a deflector for changinga direction of each laser light. Laser light emitted from the laserlight source is split into two or more optical paths so as to irradiatea screen from two directions.

In one embodiment, the laser light source includes: an opticalwavelength conversion element for generating a harmonic wave; and asingle mode fiber for conveying laser light from the semiconductor laserto the optical wavelength conversion element.

In one embodiment, the laser light source includes: the semiconductorlaser; a fiber for conveying laser light from the semiconductor laser; asolid state laser crystal for receiving laser light emitted from thefiber so as to generate a fundamental wave; and an optical wavelengthconversion element for generating a harmonic wave from the fundamentalwave.

In one embodiment, the semiconductor laser is a distributed feedbacktype semiconductor laser; and the laser light source further includes asemiconductor laser amplifier for amplifying laser light from thedistributed feedback type semiconductor laser.

In one embodiment, the laser light source includes an optical wavelengthconversion element in which an optical waveguide for guiding laser lightfrom the semiconductor laser and periodic domain inverted structures areformed. The width and the thickness of the optical waveguide are each 40μm or greater.

In one embodiment, two optical paths are formed by two laser lightsources; and the laser light sources respectively experience differentmodulations.

In one embodiment, the two optical paths are switched with each otherbased on time.

Still another laser device of the present invention includes at leastone laser light source including a semiconductor laser; a first opticalsystem for setting laser light emitted from the laser light source intoa parallel beam; a liquid crystal cell for spatially modulating theparallel beam; and a second optical system for irradiating a screen withlight emitted from the liquid crystal cell.

In one embodiment, the laser light source includes: an opticalwavelength conversion element for generating a harmonic wave; and asingle mode fiber for conveying laser light from the semiconductor laserto the optical wavelength conversion element.

In one embodiment, the laser light source includes: the semiconductorlaser; a fiber for conveying laser light from the semiconductor laser; asolid state laser crystal for receiving laser light emitted from thefiber so as to generate a fundamental wave; and an optical wavelengthconversion element for generating a harmonic wave from the fundamentalwave.

In one embodiment, the semiconductor laser is a distributed feedbacktype semiconductor laser; and the laser light source further includes asemiconductor laser amplifier for amplifying laser light from thedistributed feedback type semiconductor laser.

In one embodiment, the laser light source includes an optical wavelengthconversion element in which an optical waveguide for guiding laser lightfrom the semiconductor laser and periodic domain inverted structures areformed. The width and the thickness of the optical waveguide are each 40μm or greater.

In one embodiment, the sub-semiconductor laser is an infrared laser.

In one embodiment, laser light radiation is terminated by shifting aphase-matched wavelength of the optical wavelength conversion element.

An optical disk apparatus of the present invention includes: a laserlight source for generating laser light; an optical wavelengthconversion element for converting a fundamental wave to a harmonic wave;an optical pickup incorporating therein the optical wavelengthconversion element; and an actuator for moving the optical pickup. Thelaser light radiated from the laser light source is incident upon theoptical pickup via an optical fiber.

In one embodiment, the laser light source includes a semiconductor laserdisposed outside the optical pickup.

In one embodiment, the laser light source further includes a solid statelaser crystal for generating a fundamental wave using laser lightemitted from the semiconductor laser as pumped light.

In one embodiment, the solid state laser crystal is disposed outside theoptical pickup; and the fundamental wave generated by the solid statelaser medium is incident upon the optical wavelength conversion elementvia the optical fiber.

In one embodiment, the solid state laser crystal is disposed inside theoptical pickup; and the laser light emitted from the semiconductor laseris incident upon the solid state laser via the optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a conventional short wavelength lightsource.

FIG. 2 is a diagram illustrating a structure of a conventional opticalwavelength conversion element.

FIG. 3 is a flow chart illustrating steps of a method for producing anoptical wavelength conversion element according to a conventionalmethod.

FIG. 4 is a graph illustrating temporal variation of a harmonic waveoutput of the conventional optical wavelength conversion element.

FIG. 5 is a graph illustrating temporal variation of a phase-matchedwavelength of the conventional optical wavelength conversion element.

FIG. 6 is a graph illustrating temporal variation of a refractive indexof a conventional optical element.

FIG. 7 is a diagram illustrating a structure of an optical wavelengthconversion element according to Example 1 of the present invention.

FIGS. 8A, 8B, 8C, 8D and 8E are diagrams illustrating respective stepsof a method for producing the optical wavelength conversion elementaccording to Example 1 of the present invention.

FIG. 9 is a flow chart illustrating steps of a method for producing theoptical wavelength conversion element according to Example 1 of thepresent invention.

FIG. 10 is a characteristic diagram illustrating phase-matchedwavelength variation with respect to the annealing time, with theannealing time being a parameter.

FIG. 11 is a characteristic diagram illustrating the relationshipbetween the annealing temperature and the amount of phase-matchedwavelength variation.

FIG. 12 is a graph illustrating the output-time characteristic of theoptical wavelength conversion element according to Example 1 of thepresent invention.

FIG. 13 is a graph illustrating temporal characteristics of thephase-matched wavelength and the effective refractive index of theoptical wavelength conversion element according to Example 1 of thepresent invention.

FIG. 14 is a flow chart illustrating steps of a method for producing anoptical wavelength conversion element according to Example 2 of thepresent invention.

FIGS. 15A, 15B and 15C are diagrams illustrating respective steps of amethod for producing an optical element according to Example 4 of thepresent invention.

FIG. 16 is a flow chart illustrating steps of a method for producing anoptical element according to Example 5 of the present invention.

FIG. 17 is a diagram illustrating a configuration of an example of alaser light source according to the present invention.

FIGS. 18A, 18B, 18C and 18D are diagrams illustrating respectiveproduction steps of the optical wavelength conversion element in thelaser light source of the present invention.

FIG. 19 is a graph illustrating the relationship between the opticalwaveguide thickness and the endurance property against optical damage ofthe optical wavelength conversion element used in the laser light sourceof the present invention.

FIG. 20 is a diagram illustrating a configuration of a laser deviceaccording to an example of the present invention.

FIG. 21 is a diagram illustrating a configuration of a laser lightsource according to an example of the present invention.

FIG. 22 is a diagram illustrating a configuration of a semiconductorlaser used for a laser light source according to an example of thepresent invention.

FIG. 23 is a diagram illustrating a configuration of a laser lightsource according to an example of the present invention.

FIG. 24 is a diagram illustrating a configuration of a laser lightsource according to an example of the present invention.

FIG. 25 is a diagram illustrating a configuration of a laser lightsource of a separate type according to an example of the presentinvention.

FIG. 26 is a diagram illustrating a configuration of a laser lightsource according to an example of the present invention.

FIG. 27 is a diagram illustrating a configuration of a laser deviceaccording to an example of the present invention.

FIG. 28 is a diagram illustrating a configuration of an automaticshutdown device for a laser device according to an example of thepresent invention.

FIG. 29 is a diagram illustrating a control system for the automaticshutdown device for a laser device according to an example of thepresent invention.

FIG. 30 is a diagram illustrating a configuration of a laser deviceaccording to an example of the present invention.

FIG. 31 is a diagram illustrating a configuration of a laser deviceaccording to an example of the present invention.

FIG. 32 is a diagram illustrating a configuration of a laser deviceaccording to an example of the present invention.

FIG. 33 is a diagram illustrating a configuration of an optical diskapparatus according to an example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors of the present invention studied, with respect to theabove-described optical wavelength conversion element having an opticalwaveguide, the cause of why the phase-matched wavelength thereof becomesshorter with the passage of time, whereby a harmonic wave cannot begenerated.

FIG. 4 illustrates the relationship in the conventional opticalwavelength conversion element between the elapsed time immediately afterthe production of the element and the harmonic wave output thereof. Itcan be seen that the harmonic wave output rapidly decreases with thepassage of time.

FIG. 5 illustrates the relationship between the elapsed time and thephase-matched wavelength. The harmonic wave output decreases to halfafter 3 days from immediately after the production of the element. Itcan be seen that, at this point of time, the phase-matched wavelengthhas shifted toward the short wavelength side. The phase-matchedwavelength λ is defined by a domain inversion pitch Λ and effectiverefractive indices n_(2W) and n_(W) respectively for a harmonic wave anda fundamental wave. More particularly, λ=2 (n_(2W)−n_(W))·Λ.

Since the pitch Λ of domain inverted layers does not vary with time butis kept constant, the decrease in the phase-matched wavelength λ isconsidered to result from variation in the effective refractive indicesn_(2W) and n_(W).

FIG. 6 illustrates the relationship between the effective refractiveindex n_(2W) and the elapsed time. It can be seen from FIG. 6 that theeffective refractive index n_(2W) decreases as more days elapse from theday when the element was produced.

The inventors of the present invention consider the cause therefor to beas follows.

The high temperature treatment at about 400° C. which is performed whenforming an optical waveguide introduces some strain, or the like, into aproton exchange layer, whereby a layer with an increased refractiveindex (the altered layer) is formed in the proton exchange layer. Thestrain is released gradually with the passage of time, so that therefractive index of the altered layer becomes closer to the originalrefractive index thereof.

Although the altered layer with an increased refractive index is formeddue to the strain, or the like, which is generated during the hightemperature annealing, the refractive index of the altered layer returnsto the original magnitude thereof and, eventually, the altered layerbecomes a stable proton exchange layer. However, it takes years for thealtered layer to become such a stable proton exchange layer. In thespecification of the present application, a proton exchange layer whoseeffective refractive index does not decrease with time, when used at anordinary temperature (about 0° C. to about 50° C.), is referred to as a“stable proton exchange layer”.

The above is the mechanism for the temporal variation suggested by theinventors of the present invention. In order to confirm this, a samplewhose refractive index has lowered due to the temporal variation wasannealed at 300° C. for 1 minute. Such annealing temperature andannealing time will scarcely cause diffusion of proton, etc., and thewaveguide will not be widened. Therefore, from the conventional point ofview, the refractive index of the proton exchange layer should not varyat all. However, in an experiment by the inventors, the refractive indexincreased again by the annealing at 300° C. for 1 minute. Moreover, aphenomenon was observed where the refractive index decreased again withthe passage of time after this annealing.

The present invention makes it possible to mitigate the strain generatedin the proton exchange layer due to a heat treatment at a relativelyhigh temperature, and thus to prevent the temporal variation of theoptical wavelength conversion element.

Hereinafter, examples will be described with reference to theaccompanying drawings.

EXAMPLE 1

Referring to FIG. 7, Example 1 of the present invention will bedescribed.

In an optical wavelength conversion element of the present example, anoptical waveguide of a stable proton exchange layer is formed in anLiTaO₃ substrate 1, and a plurality of domain inverted layers 3 areperiodically arranged along the optical waveguide. By making afundamental wave P1 incident upon an input end of the optical waveguide,a harmonic wave P2 is emitted from an output end thereof. The length ofthe optical wavelength conversion element (length of the opticalwaveguide) is 9 mm in the present example. Moreover, in order to allowfor operation at a wavelength of 850 nm, the length of a pitch of thedomain inverted layers 3 is set to 3.7 μm.

Hereinafter, referring to FIGS. 8A to 8E, a production method of theoptical wavelength conversion element will be described.

First, as shown in FIG. 8A, a Ta film is deposited so as to cover theprincipal surface of the LiTaO₃ substrate 1, after which ordinaryphotolithography and dry etching techniques are used to pattern the Tafilm (thickness: about 200 to 300 nm) into a striped pattern, therebyforming the Ta mask 6. The Ta mask 6 used in the present example has apattern where strips each 1.2 μm wide and 10 mm long are arranged so asto be equally spaced apart from one another, and the arrangement pitchof the strips is 3.7 μm. A proton exchange process is performed for theLiTaO₃ substrate 1 whose principal surface is covered by the Ta mask 6.The proton exchange process is performed by immersing the surface of thesubstrate 1 for 14 minutes in a pyrophosphoric acid heated to 230° C.Thus, 0.5 μm thick proton exchange layers 7 are formed in portions ofthe LiTaO₃ substrate 1 which are not covered by the Ta mask. Then, theTa mask is removed by etching for 2 minutes using a mixture containingHF:HNF₃ at 1:1.

Next, as shown in FIG. 8B, a domain inverted layer is formed in each ofthe proton exchange layers 7 by performing a heat treatment at atemperature of 550° C. for 15 seconds. In the heat treatment, thetemperature rise rate is 50 to 80° C./sec and the cooling rate is 1 to50° C./sec. In portions of the LiTaO₃ substrate 1 where the protonexchange has been performed, the amount (concentration) of Li is reducedas compared to that in other portions thereof where the proton exchangehas not been performed. Therefore, the Curie temperature of the protonexchange layer 7 decreases as compared to that in other portions,whereby the domain inverted layer 3 can be formed partially in theproton exchange layer by a heat treatment at a temperature of 550° C.This heat treatment allows for formation of the domain inverted layers 3having a periodic pattern reflecting the pattern of the Ta mask 6.

Next, the Ta mask (not shown) used for forming the optical waveguide isformed. The Ta mask is obtained by forming slit-shaped openings (width:4 μm, length: 12 mm) in a Ta film (thickness: about 200 to 300 nm)deposited on the substrate 1. The openings define the planar layout ofthe waveguide. It is needless to say that the shape of the waveguide isnot limited to the linear shape. The pattern of the Ta mask isdetermined depending upon the shape of a waveguide to be formed. Byperforming a proton exchange process at 260° C. for 16 minutes withrespect to the LiTaO₃ substrate 1 covered by the Ta mask, alinearly-extending proton exchange layer (thickness: 0.5 μm, width: 5μm, length 10 mm) 5 is formed in a region of the LiTaO₃ substrate 1under an opening of the Ta mask, as shown in FIG. 8C. Thelinearly-extending proton exchange layer 5 will eventually function as awaveguide. Then, the Ta mask is removed by etching for 2 minutes using amixture containing HF:HNF₃ at 1:1.

Next, an infrared heating equipment is used to perform annealing at 420°C. for 1 minute. By this annealing, non-linearity of the proton exchangelayer 5 is restored, while an altered layer 8 b where the refractiveindex is increased by about 0.03 is formed, as shown in FIG. 8D. Asdescribed above, this annealing serves to allow Li and proton to bediffused in the substrate 1 and to reduce the proton exchangeconcentration of the proton exchange layer 5. Thereafter, a 300 nm thickSiO₂ layer (not shown), which functions as a protective layer, isdeposited on the principal surface of the substrate 1.

Next, after the surface of the substrate 1 perpendicular to the alteredlayer 8 b is optically polished so as to form an incident portion and anemitting portion of the optical wavelength conversion element, anantireflection (AR) coating 15 is formed on the polished surface of theincident portion and the emitting portion, as shown in FIG. 8E.

Next, low-temperature annealing is performed for preventing temporalvariation. In the specification of the present application,“low-temperature annealing” means a heat treatment performed at atemperature which does not substantially reduce the proton concentrationin the proton exchange layer. For example, in the case of the LiTaO₃substrate, “low-temperature annealing” means a heat treatment performedat a temperature of about 130° C. or lower. In the present example, aheat treatment is performed at 60° C. for 40 hours under an airatmosphere using an oven. By such low-temperature annealing, a stableproton exchange layer 8 a is formed. The stable proton exchange layer 8a forms the optical waveguide.

Referring to FIG. 9, the flow of the above-described production stepswill be described.

After a step of forming the domain inverted layers in the substrate(step S10), an optical waveguide formation step (S20) is performed. Theoptical waveguide formation step (S20) is generally divided into stepS21, step S22 and step S23. The mask pattern is formed at step S21; theproton exchange process is performed at step S22; and high-temperatureannealing is performed at step S23. Then, a protective film formationstep (step S30), an end face polishing step (step S40), an AR coatingstep (step S50) are performed. Since the wavelength conversion element,as thus formed, will have some temporal variation, low-temperatureannealing is performed at step S60 so as to form a stable protonexchange layer.

FIG. 10 illustrates the relationship with the annealing time in caseswhere low-temperature annealing is performed respectively at 60° C. and120° C. The amount of phase-matched wavelength shift becomessubstantially constant after a few hours in the case of annealing at120° C., but it takes some ten hours to become substantially constant inthe case of annealing at 60° C.

It can be seen from FIG. 10 that a steady state is achieved in a shorterannealing time as the temperature of the low-temperature annealing ishigher. Moreover, as the annealing temperature is lower, the amount ofphase-matched wavelength shift when achieving the steady state indicatesa value closer to zero. Thus, if the temperature of the low-temperatureannealing is increased, a period of time required for the shift amountto return to zero becomes shorter, but, on the other hand, a relativelygreat strain will remain.

FIG. 11 illustrates the relationship between the amount of phase-matchedwavelength shift when a steady state is achieved and the temperature oflow-temperature annealing. It can be seen from FIG. 11 that thephase-matched wavelength becomes stable in the condition where beingshifted by about 0.5 nm when annealing is performed at 120° C. Ifannealing is performed at 150° C. or higher, the amount of phase-matchedwavelength shift after stabilization is 0.8 nm or more. If aphase-matched wavelength shift of such a magnitude remains, long-termuse of the optical wavelength conversion element becomes difficult. Ifthe tolerance range of the phase-matched wavelength shift is set to be0.5 nm or less, the amount of shift cannot be reduced to fall within thetolerance range even if annealing is performed at a temperatureexceeding 120° C. When the tolerance range of the phase-matchedwavelength shift is extended, the conversion efficiency is reduced. Whenthe amount of the phase-matched wavelength shift exceeds 0.5 nm, only anoutput about ¼ of that obtainable when the shift amount is zero isobtained. If low-temperature annealing is performed at 60° C., theannealing time is longer, but the shift amount can be reduced to be 0.1nm or less, whereby the problem of the reduced conversion efficiency iseliminated. It is preferable to suppress the amount of shift in thephase-matched wavelength to be about 0.2 nm or less.

According to the present example, the respective refractive indices ofthe domain non-inverted layer 4 and the domain inverted layers 3 in theoptical waveguide 2 have no temporal variation, and the propagation lossas light is being guided is small. When laser light (wavelength: 850 nm)from a semiconductor laser was made incident upon the incident portionas the fundamental wave P1 so as to propagate through the opticalwaveguide, the light propagated in a single mode, and the harmonic wave.P2 having a wavelength of 425 nm was taken out of the substrate throughthe emitting portion. The harmonic wave P2 was effectively obtained witha small propagation loss of 1 dB/cm in the optical waveguide 2. For aninput of a fundamental wave of 27 mW, a harmonic wave (wavelength: 425nm) of 1.2 mW was obtained. In this case, the conversion efficiency is4.5%.

FIG. 12 illustrates the relationship between the number of days elapsedand the harmonic wave output. FIG. 13 illustrates the relationshipbetween the number of days elapsed and the phase-matched wavelength and,as well as the relationship between the number of days elapsed and therefractive index variation.

It can be seen from these figures that the refractive index variationand the phase-matched wavelength become constant immediately afterproduction of the element. According to the production method of theoptical wavelength conversion element of the present invention, it waspossible to realize an optical wavelength conversion element which hasno refractive index variation with the passage of time and which thushas a constant phase-matched wavelength. By combining this element witha semiconductor laser, it is possible to produce a stable shortwavelength laser. At a temperature of about 60° C., low-temperatureannealing for 40 hours or more is particularly effective.

EXAMPLE 2

Next, Example 2 of the present invention will be described.

First, a Ta film is deposited so as to cover the principal surface ofthe LiTaO₃ substrate, after which ordinary photolithography and dryetching techniques are used to pattern the Ta film (thickness: about 200to 300 nm) into a striped pattern, thereby forming the Ta mask. The Tamask used in the present example has a pattern where strips each 1.2 μmwide and 10 mm long are arranged so as to be equally spaced apart fromone another, and the arrangement pitch of the strips is 3.6 μm. A protonexchange process is performed for the LiTaO₃ substrate 1 whose principalsurface is covered by the Ta mask. The proton exchange process isperformed by immersing the surface of the substrate for 20 minutes in apyrophosphoric acid heated to 260° C. Thus, 0.5 μm thick proton exchangelayers are formed in portions of the LiTaO₃ substrate 1 which are notcovered by the Ta mask. Then, the Ta mask is removed by etching for 2minutes using a mixture containing HF:HNF₃ at 1:1.

Next, a domain inverted layer is formed in each of the proton exchangelayers 7 by performing a heat treatment at a temperature of 550° C. for15 seconds. In the heat treatment, the temperature rise rate is 50°C./sec and the cooling rate is 10° C./sec. This heat treatment allowsfor formation of the domain inverted layers having a periodic patternreflecting the periodic pattern of the Ta mask.

Referring to FIG. 14, the flow of the steps following theabove-described steps will be described.

First, a proton exchange process is performed for the surface of thesubstrate on which the domain inverted layers are arranged so as to forman optical waveguide (step S100). A Ta film, in which slits each 4 μmwide and 12 mm long are formed, is used as a mask for forming theoptical waveguide.

Next, proton exchange is performed at 260° C. for 16 minutes in apyrophosphoric acid (step S110), after which the Ta mask is removed.After covering the principal surface of the substrate with an SiO₂ filmhaving a thickness of 300 nm, low-temperature annealing (step S120) isperformed so as to complete the formation of the optical waveguide. Forthe low-temperature annealing, a heat treatment in air at 120° C. wasperformed for 200 hours in order to prevent the refractive index fromincreasing. By this low-temperature annealing, a stable proton exchangelayer is formed.

Through the above-described steps, the domain inverted layers and theoptical waveguide are formed in the substrate. When the thickness of thedomain inverted layer is set to 2.2 μm, in order to effectively performwavelength conversion, the thickness d of the optical waveguide is setto be thinner than the thickness of the domain inverted layer, e.g., 1.8μm. In order to allow for operation at a wavelength of 840 nm, the pitchof the domain inverted layers is set to 3.6 μm.

According to the above-described production method, the respectiverefractive indices of the domain non-inverted layer and the domaininverted layers have no temporal variation, and the propagation loss oflight is small. The surface perpendicular to the optical waveguide wasoptically polished so as to form an incident portion and an emittingportion. Thus, an optical wavelength conversion element can be produced.Moreover, the length of the element is 9 mm.

When semiconductor laser light (wavelength: 840 nm) as the fundamentalwave P1 was made incident upon the incident portion of the waveguide,the harmonic wave P2 having a wavelength of 420 nm was taken out of thesubstrate through the emitting portion. A harmonic wave (wavelength: 420nm) having an output of 10 mW was obtained for an input of a fundamentalwave having an output of 80 mW. In this case, the conversion efficiencyis 12%. The harmonic wave output was very stable with no optical damageor no temporal variation. When a high-temperature annealing step is notperformed in the course of the process, as in this example, the temporalvariation can be prevented.

EXAMPLE 3

Next, as Example 3 of the present invention, a case of using an LiNbO₃substrate (thickness: 0.4 to 0.5 mm) will be described.

First, ordinary photolithography and dry etching techniques are used toform a Ta electrode (first Ta electrode) having a pattern similar to thepattern of the Ta mask used in the above-described examples on theprincipal surface of the LiNbO₃ substrate.

Then, a Ta film (second Ta electrode) is deposited on the entire reversesurface of the substrate. The first Ta electrode formed on the principalsurface of the substrate and the second Ta electrode formed on thereverse surface of the substrate form an electrode structure forapplying an electric field across the substrate.

Next, a voltage (e.g., 10 kilovolts) is applied between the first Taelectrode and the second Ta electrode so as to form an electric field inthe LiNbO₃ substrate. By the application of an electric field, a domaininverted layer is formed so as to extend from a portion of the surfaceof the substrate being in contact with the first Ta electrode to thereverse surface of the substrate.

Next, etching is performed for 2 minutes using a mixture containingHF:HNF₃ at 1:1 so as to remove the Ta electrode. Then, a Ta mask havingslit-shaped openings (width: 4 μm, length: 12 mm) is formed on thesubstrate, after which a proton exchange process (230° C., 10 minutes)using a pyrophosphoric acid is performed so as to form an opticalwaveguide. After removing the Ta mask, annealing at 420° C. for 2minutes is performed using infrared heating equipment. By thisannealing, non-linearity in the optical waveguide is restored, but analtered layer is formed where the refractive index is increased by about0.02.

Then, a 300 nm thick SiO₂ film, which functions as a protective film, isdeposited on the substrate. Next, in order to mitigate the strain whichcauses the refractive index to increase, annealing in air at 100° C. for20 hours (first stage low-temperature annealing) is performed, which isfollowed by annealing at 60° C. for 10 hours (second stagelow-temperature annealing). Thus, two stages of low-temperatureannealing are performed in the present example. The low-temperatureannealing is performed in separate two stages in order to reduce thetotal amount of time required for the low-temperature annealing. Byannealing at 100° C., the strain is mitigated more quickly than inannealing at 60° C., but some strain remains which corresponds to theamount of the phase-matched wavelength shift at 100° C. as shown in FIG.11. Therefore, low-temperature annealing at 60° C. is additionallyperformed so as to completely eliminate the strain. This 2-stageannealing makes it possible to quickly and completely form the “stableproton exchange layer” which is unlikely to generate strain.

The thickness d of the optical waveguide formed by the steps asdescribed above is 1.8 μm. The arrangement pitch of the domain invertedlayers is 3 μm, and it operates at a wavelength of 840 nm. The surfaceperpendicular to the optical waveguide is optically polished so as toform the incident portion and the emitting portion. Thus, the opticalwavelength conversion element can be produced. Moreover, the length ofthe element is 10 mm. When semiconductor laser light (wavelength: 840nm) as the fundamental wave P1 was guided from the incident portion, theharmonic wave P2 having a wavelength of 420 nm was taken out of thesubstrate through the emitting portion. A harmonic wave (wavelength: 420nm) of 13 mW was obtained for an input of a fundamental wave of 80 mW.The harmonic wave output was very stable with no temporal variation.

Although two different low-temperature annealings at differenttemperatures (2-stage annealing) were performed in this example, it isalso applicable to perform low-temperature annealing where thetemperature is gradually lowered, for example, from 100° C. to 60° C. in30 hours.

EXAMPLE 4

Next, referring to FIGS. 15A to 15C, Example 4 of the present inventionwill be described.

First, as shown in FIG. 15A, a mixture film (LiNb_(0.5)Ta_(0.5)O₃ film)16′ of LiNbO₃ and LiTaO₃ is grown on the LiTaO₃ substrate 1 by a liquidphase epitaxial growth method. At this point of time, the growthtemperature exceeds 1000° C., and some strain remains at the interfacebetween the mixture film 16 and the LiTaO₃ substrate 1. Next, as shownin FIG. 15B, a resist mask 17 is formed on the mixture film 16′ using anordinary photolithography technique. Next, as shown in FIG. 15C, aportion of the mixture film 16 which is not covered with the resist mask17 is removed by ion beam etching so as to leave the optical waveguide16 having a width of, for example, 4 μm.

After a 300 nm thick SiO₂ is deposited on the substrate 1 by a vapordeposition method, low-temperature annealing is performed in order tomitigate an increase in the refractive index. This annealing includes afirst stage low-temperature annealing performed at 100° C. for 30 hoursand a subsequent low-temperature annealing performed at 70° C. for 60hours. By this low-temperature annealing, a stable optical waveguide 16with no refractive index variation is obtained.

The thickness d of the optical waveguide formed by the above-describedsteps is 1.8 μm. Moreover, the length of the element is 9 mm. Thesurface perpendicular to the optical waveguide was optically polished soas to form the incident portion and the emitting portion. Whensemiconductor laser light (wavelength: 840 nm) was guided from theincident portion, the waveguide loss was very small. It was very stablewith the temporal variation of the refractive index being less than themeasuring limit. The material of the mixture film is not limited toLiNb_(0.5)Ta_(0.5)O₃, but may also be LiNb_(x)Ta_(1-x)O₃ (0<x<1) or anyother optical material.

EXAMPLE 5

Next, Example 5 of the present invention will be described.

Referring to FIG. 16, the outline of the process flow of the presentexample will be described.

First, an optical waveguide formation step is performed. The opticalwaveguide formation step is generally divided into step S200, step S210and step S220. The mask pattern is formed at step S200; the protonexchange process is performed at step S210; and high-temperatureannealing is performed at step S220. Then, an electrode formation step(step S230), a low-temperature annealing step (step S240), an end facepolishing step (step S250) and an AR coating step (step S260) areperformed.

Hereinafter, details of the process will be described.

First, ordinary photo process and dry etching are used to pattern Tainto slits. Next, proton exchange is performed at 30° C. for 10 minutesfor the LiTaO₃ substrate 1, on which the pattern of Ta has been formed,so as to form a 0.5 μm thick proton exchange layer directly under theslit. Next, Ta is removed by etching for 2 minutes using a mixturecontaining HF:HNF₃ at 1:1. A diffusion furnace is used to performannealing (first annealing) at 400° C. for 1 hour, and an altered layeris formed where the refractive index is increased by about 0.01. Next,as the electrode formation step, 300 nm of SiO₂ was added by vapordeposition. After Al was deposited into a striped shape as an electrodemask, patterning was performed. Next, low-temperature annealing wasperformed in order to mitigate an increase in refractive index.Annealing was performed in air at 70° C. for 10 hours. Thus, a stableproton exchange layer is formed. Herein, the second annealing wasperformed at a temperature lower than that in the first annealing by330° C. Lowering it by 200° C. or more is effective because the straincan be greatly mitigated thereby. Finally, polishing and AR coating wereperformed.

By the steps as described above, an optical waveguide with an electrodewas produced. This functions as an optical modulator. The thickness ofthe optical waveguide is 8 μm. The surface perpendicular to the opticalwaveguide was optically polished so as to form the incident portion andthe emitting portion. Thus, an optical element can be produced.Moreover, the length of the element is 9 mm. When a modulation signal isapplied to the electrode so as to guide semiconductor laser light(wavelength: 1.56 μm) as a fundamental wave from the incident portion,modulated light was taken out through the emitting portion. There was notemporal variation, and the bias voltage remained stable for more than2000 hours.

Although the present invention has been described in respect of anoptical wavelength conversion element and an optical modulator as anexample of an optical element in the above-described examples, thepresent invention is not limited thereto, but may also be applied to aflat device such as a Fresnel lens or a hologram. Temporal variation inrefractive index associated with the proton exchange process can beprevented while deterioration of characteristics can be suppressed.

EXAMPLE 6

Next, referring to FIG. 17, Example 6 of the present invention will bedescribed. The present example is a short wavelength light sourceincluding a semiconductor laser and an optical wavelength conversionelement.

As shown in FIG. 17, the pumped light P1 a emitted from thesemiconductor laser 20 is collected by the lens 30 so as to excite theYAG 21 as a solid-state laser crystal.

The total reflection mirror 22 for 947 nm is formed on the YAG 21,whereby laser oscillation occurs at a wavelength of 947 nm so as toradiate the fundamental wave P1. On the other hand, the total reflectionmirror 23 for the fundamental wave P1 is formed on the emitting side ofthe optical wavelength conversion element 25, whereby laser oscillationoccurs therebetween. The fundamental wave P1 is collected by a lens 31,and the fundamental wave P1 is converted to the harmonic wave P2 by theoptical wavelength conversion element 25. In this example, the opticalwaveguide 2 produced in the LiTaO₃ substrate 1 by utilizing protonexchange is used as an optical wavelength conversion element havingperiodic domain inverted structures where a periodic structure isformed.

In FIG. 17, reference numeral 1 denotes an LiTaO₃ substrate of a Zplate; 2 denotes a formed optical waveguide; 3 denotes a domain invertedlayer; 10 denotes an incident portion for the fundamental wave P1; and12 denotes an emitting portion for the harmonic wave P2. The fundamentalwave P1 which has entered the optical waveguide 2 is converted to theharmonic wave P2 by the domain inverted layer 3 which has a length ofthe phase-matched length L, and the harmonic wave power is thenincreased by the domain non-inverted layer 4 which also has the lengthof L.

In this manner, the harmonic wave P2 whose power has been increased inthe optical waveguide 2 is radiated from the emitting portion 12. Theradiated harmonic wave P2 is collimated by the a lens 32.

Moreover, an electrode 14 is formed in the optical wavelength conversionelement 25 via a protective film 13. Next, the production method of theoptical wavelength conversion element 25 will be briefly describedreferring to the figures.

First, as shown in FIG. 18A, ordinary photolithography and dry etchingtechniques are used to form a Ta electrode (first Ta electrode) 6 havinga pattern similar to the pattern of the Ta mask used in theabove-described respective examples on the principal surface of the 0.3mm thick LiNbO₃ substrate 1.

Then, a Ta film (second Ta electrode) 6 b is deposited on the entirereverse surface of the substrate 1. The first Ta electrode 6 formed onthe principal surface of the substrate 1 and the second Ta electrode 6 bformed on the reverse surface of the substrate 1 form an electrodestructure for applying an electric field across the substrate 1.

Next, a voltage (e.g., 10 kilovolts) is applied between the first Taelectrode 6 and the second Ta electrode 6 b so as to form an electricfield in the LiNbO₃ substrate 1. By the application of an electricfield, a domain inverted layer 3 is formed so as to extend from aportion of the surface of the substrate 1 in contact with the first Taelectrode 6 to the reverse surface of the substrate 1, as shown in FIG.18B. The length L of the domain inverted layer 3 along the direction inwhich light propagates is 2.5 μm. Then, etching is performed for 20minutes using a mixture containing HF:HNF₃ at 1:1 so as to remove the Taelectrodes 6 and 6 b.

Then, a Ta mask (not shown) having slit-shaped openings (width: 4 μm,length: 12 mm) is formed on the substrate 1, after which a protonexchange process (260° C., 40 minutes) using a pyrophosphoric acid isperformed so as to form the optical waveguide 2, as shown in FIG. 18C.The Ta mask has slits (width: 6 μm, length: 10 mm), and the slits definethe planar layout of the optical waveguide 2. After removing the Tamask, annealing for 5 hours at 460° C. is performed using infraredheating equipment. By this annealing, the optical waveguide for whichproton exchange has been performed restores its non-linearity, and therefractive index at the portion increases by about 0.002. Lightpropagates along the optical waveguide 2 having a high refractive index.The thickness d of the optical waveguide 2 is 50 μm, and the widththereof is 70 μm. The arrangement pitch of the domain inverted layers 3along the direction in which the waveguide 2 extends is 5 μm, and theoptical wavelength conversion element operates for a fundamental wavehaving a wavelength of 947 nm.

Next, as shown in FIG. 18D, after a protective film (thickness: 300 to400 nm) 13 made of SiO₂ is formed on the substrate 1, an Al film(thickness: 200 nm) is formed on the protective layer 13 by vapordeposition. The Al film is patterned by photolithography technique so asto form the Al electrode 14. The Al electrode 14 is used for modulatingthe intensity of output light.

The surface perpendicular to the direction in which the opticalwaveguide 2 extends is optically polished so as to form the incidentportion 10 and the emitting portion 12 as shown in FIG. 17. Moreover, anantireflection coating for the fundamental wave P1 is applied onto theincident portion 10. A reflective coating (99%) for the fundamental waveP1 and an antireflection coating for the harmonic wave P2 are appliedonto the emitting portion 12.

In this way, the optical wavelength conversion element 25 (elementlength: 10 mm) as shown in FIG. 17 can be produced.

In FIG. 17, when light having a wavelength of 947 nm as the fundamentalwave P1 was guided from the incident portion 10, it propagated in asingle mode, and the harmonic wave P2 having a wavelength of 473 nm wastaken out of the substrate through the emitting portion 12. Thepropagation loss in the optical waveguide 2 was as small as 0.1 dB/cm,thus improving the performance of the cavity, increasing the powerconcentration of the fundamental wave P1, and generating the harmonicwave P2 at a high efficiency.

Possible causes for the reduced loss may include that a uniform opticalwaveguide was formed by the phosphoric acid and that the confinement inthe waveguide was reduced. Moreover, due to the optical waveguide withthe weak confinement, the harmonic wave concentration was reduced, andthe optical damage was considerably improved. This is because, an area100 times larger with respect to that in a conventional technique cantolerate a 100 times greater optical damage.

FIG. 19 illustrates the relationship between the optical waveguidethickness and the endurance power against optical damage. The endurancepower against optical damage is a power that indicates the highest blueharmonic wave which can be tolerated, i.e., the highest blue harmonicwave for which optical variation does not occur. It can be seen that,when the thickness of the optical waveguide is widened, the widththereof is also simultaneously widened due to diffusion, whereby theendurance power against optical damage is improved in relation to theoptical waveguide thickness being substantially squared. Since the powerrequired for laser radiation is at least 2 W, it is preferable that theoptical waveguide thickness is 40 μm or greater.

Moreover, if the cross-section of the waveguide is enlarged in the casewhere the refractive index distribution in and in the vicinity of thewaveguide varies in a stepped manner, a multi-mode propagationphenomenon occurs. In order to avoid this, a waveguide having a gradedtype refractive index distribution is formed in the present example.

When the output of the output light P1 a from the semiconductor laser 20was 10 W, a harmonic wave P3 having an output of 3 W was obtained. Inthis case, the conversion efficiency is 30%. The tolerance range of theoptical wavelength conversion element against the wavelength variationis 0.4 nm. Even if the wavelength is shifted by 0.4 nm, the oscillationwavelength of the solid state laser was constant, while the harmonicwave output was stable. By applying a voltage to the Al electrode 14 formodulation, the refractive index varies in and in the vicinity of thewaveguide, thereby shifting the phase-matched wavelength of the opticalwavelength conversion element. By utilizing the phenomenon that thephase-matched wavelength is greatly shifted by a voltage application, itis possible to modulate the harmonic wave output with application of arelatively low voltage of about 100 V.

Thus, with the optical wavelength conversion element using the periodicdomain inverted structures used in the present example, it is possibleto easily modulate the harmonic wave output by applying a voltage, and avoltage required to be applied is thus low, providing high industrialapplicability.

Thus, a modulator can be integrated, whereby it is possible to achievesmaller size, lighter weight and lower cost. Moreover, there is anotherfeature that LiTaO₃, which is a non-linear optical crystal used in thepresent invention, can be obtained in a large crystal, whereby it iseasy to mass produce the optical wavelength conversion element using anoptical IC process. Multi-mode propagation for a fundamental waveresults in an unstable harmonic wave output and is thus unpractical,whereas a single mode is effective. It is highly desirable to use anelement having periodic domain inverted structures as the opticalwavelength conversion element, as in this example, since this makes itpossible to improve efficiency and realize integration of the opticalmodulator, as well as allowing red and green laser light in addition toblue laser light to be taken out by varying the pitch. The opticalmodulator may also be separated.

Next, referring to FIG. 20, an example of a laser projection apparatusof the present invention will be described. As shown in FIG. 20, theblue laser light source shown in FIG. 17 was used as the light sourcefor this laser projection apparatus. Reference numeral 45 denotes alaser light source having a wavelength in the 473 nm band which is bluecolor. The blue light is modulated by inputting a modulation signal to amodulation electrode. The blue laser light which has been modulated isincident upon a deflector. Reference numeral 56 denotes a verticaldeflector, and 57 denotes a horizontal deflector, for both of which arotating polygon mirror is used. A brightness of 300 cd/m², a contrastratio of 100:1 and a horizontal resolution of 1000 TV were obtained fora screen size of 4 m×3 m using a screen 70 having a gain of 3. Thus, theresolution was considerably improved as compared to that in aconventional technique. As compared to a configuration using a gaslaser, other considerable improvements were also achieved such as aone-thousandth weight, a one-thousandth volume and a one-hundredth powerconsumption. The small size and the low power consumption of the laserlight source and the integration of the optical modulator greatlycontribute to these achievements. That is, this results from that theconfiguration using a semiconductor laser and an optical wavelengthconversion element can be subminiaturized, and that the efficiency ofconversion from an electrical power is higher than that of a gas laserby about two orders of magnitude. Particularly, it is significantlyeffective to use an element having periodic domain inverted structuresas an optical wavelength conversion element, whereby efficiency can beimproved while the optical modulator can be integrated. Although thescreen is irradiated with laser light from the reverse side thereof inthe present example, it is also applicable to radiate it from the frontside thereof.

Next, referring to FIG. 21, another example of a laser light source ofthe present invention will be described.

As shown in FIG. 21, the fundamental wave P1 emitted from thesemiconductor laser 20 is guided to the optical wavelength conversionelement 25 via the lens 30, a half-wave plate 37 and a collective lens31, and is converted to the harmonic wave P2. That is, in this example,blue light is obtained without using a solid state laser. The structureof the optical wavelength conversion element 25 is substantially thesame as that in Example 1. The present example also uses an LiTaO₃substrate and an optical wavelength conversion element of the opticalwaveguide type. Moreover, for performing optical modulation, theelectrode 14 and the protective film 13 are formed. However, the presentexample does not employ the cavity structure.

FIG. 22 illustrates an internal structure of the semiconductor laser 20.The semiconductor laser 20 is composed of a distributed feedback type(hereinafter, abbreviated as DBR) semiconductor laser 20 a and asemiconductor laser amplifier 20 b. The DBR semiconductor laser 20 a isprovided with a DBR section 27 using a grating, and thus stablyoscillates at a constant wavelength. A stabilized fundamental wave P0emitted from the DBR semiconductor laser 20 a is guided to thesemiconductor laser amplifier 20 b by a lens 30 a. The power isamplified by an active layer 26 b of the semiconductor laser amplifier20 b so as to provide a stable fundamental wave P1. By incorporatingthis into the optical wavelength conversion element 25, the conversionefficiency and the harmonic wave output are considerably improved. Thepitch of domain inversion is 3 μm, and the optical waveguide length is 7mm. In this example, the oscillation wavelength of the semiconductorlaser was 960 nm, the wavelength of the generated harmonic wave P2 was480 nm, and the color was blue. The conversion efficiency is 10% for aninput of 10 W. There was no optical damage, and the harmonic wave outputwas very stable. A DBR semiconductor laser has a stable oscillationwavelength and is favorable in stabilizing the harmonic wave output.

Next, an RF superimposition (radio frequency superimposition) wasperformed for this DBR semiconductor laser. A pulse train was opticallyoutput from the semiconductor laser by applying a sine-shaped electricwaveform of 800 MHz to the DBR semiconductor and utilizing therelaxation oscillation. When the RF superimposition is thus performedfor the DBR semiconductor laser, the peak output of the fundamental waveis considerably improved while keeping the oscillation wavelengthconstant. For a fundamental wave with an average output of 10 W, aharmonic wave of 5 W was obtained with a conversion efficiency of 50%.The conversion efficiency was improved by 5-fold as compared to the casewhen the RF superimposition is not performed.

Although the DBR semiconductor laser and the semiconductor laseramplifier were separated from each other in the present example, furtherminiaturization can be achieved if they are integrated.

Next, referring to FIG. 23, still another example of the laser lightsource of the present invention will be described. The fundamental waveP1 from the semiconductor laser 20 is gently collected to the opticalwavelength conversion element 25 by the lens 30. In the present example,LiNbO₃ was used as a substrate in place of the LiTaO₃ substrate.Moreover, the bulk type optical wavelength conversion element 25 isused. The LiNbO₃ substrate 1 a has a feature of large non-linearity. Thepeak power is improved by the RF driving of the semiconductor laser 20,whereby the conversion efficiency of the optical wavelength conversionelement is considerably improved. The pitch of the domain invertedlayers 3 is 3.5 μm, and the length of the optical wavelength conversionelement 25 is 7 mm. In this example, the output of the harmonic wave P2is stabilized by using the optical feedback method. The wavelengthtolerance range of the optical wavelength conversion element 25 is asnarrow as about 0.1 nm. The fundamental wave P1 which has not beenconverted by the optical wavelength conversion element 25 is collimatedby the lens 32 and is reflected by a grating 36 so as to return to thesemiconductor laser 20. Thus, the oscillation wavelength of thesemiconductor laser 20 is locked at the reflection wavelength of thegrating 36. In order to adjust the oscillation wavelength to thephase-matched wavelength of the optical wavelength conversion element25, the angle of the grating 36 can be varied.

On the other hand, the harmonic wave P2 is reflected by a dichroicmirror 35 so as to be taken out in a different direction. In thisexample, the oscillation wavelength of the semiconductor laser was 980nm, and the harmonic wave P2 taken out was blue light at 490 nm. At thistime, an electric waveform having an RF frequency of 810 MHz and anoutput of 5 W was applied. Moreover, a harmonic wave of 3 W was obtainedfor an average output of the fundamental wave being 15 W. There was nooptical damage and the harmonic wave output was very stable. The opticaldamage is not present because the fundamental wave is collected only toabout 100 μm, and the harmonic wave is accordingly not so large in termsof concentration.

Although a wavelength is locked by optical feedback using a grating inthe present example, the present invention is not limited thereto, butit is also applicable, for example, to achieve optical feedback by usinga filter to select a wavelength. Moreover, if a laser projectionapparatus is formed using the laser light source of the present example,it is possible to achieve smaller size, lighter weight and lower cost.Furthermore, according to the present example, a harmonic wave can alsobe modulated by directly modulating the semiconductor laser, whereby theconfiguration becomes simple, and it is possible to reduce the cost.

Next, referring to FIG. 24, another example of a laser light source ofthe present invention will be described. The cross section of theoptical wavelength conversion element (bulk type) 25 is shown in FIG.24.

The pumped light P1 a emitted from the semiconductor laser 20 having awavelength of 806 nm is incident upon a fiber 40 and propagates throughthe fiber 40. The pumped light P1 a emitted from the fiber 40 enters theoptical wavelength conversion element 25. The material of the opticalwavelength conversion element 25 is an LiTaO₃ substrate 1 b into whichNd, being a rare earth element, is doped, and the domain invertedstructures are formed with a pitch of 5.1 μm. The doping amount of Nd is1 mol %. Reference numeral 22 denotes a total reflection mirror whichtotally reflects 99% of light having a wavelength of 947 nm buttransmits light in the 800 nm band. Reference numeral 23 also denotes atotal reflection mirror but which totally reflects 99% of light having awavelength of 947 nm and transmits light in the 470 nm band. Moreover,the total reflection mirror section is processed to be a sphericalshape. That is, it serves as a spherical mirror. The optical wavelengthconversion element 25 oscillates at a wavelength of 947 nm as excited bythe semiconductor laser 20, and the light is converted to the harmonicwave P2 by the domain inverted structures of the domain inverted layers3 so as to be emitted out. A harmonic wave of 2 W was obtained for thepumped light P1 of 20 W. Moreover, temperature stabilization is providedby a Peltier element so that the temperature of the optical wavelengthconversion element does not vary considerably. The conversion section ofthe laser light source according to this example has a length of 10 mm,and it can be made very compact by doping a rare earth element into theoptical wavelength conversion element and designing it so that thepumped light propagates through a fiber. Moreover, it is possible toprevent temperature variation by remotely disposing the opticalwavelength conversion element away from the heat generated by thesemiconductor laser.

Furthermore, by changing the coating on the total reflection mirrors 22and 23 for reflection of the 1060 nm band, and changing the pitch of thedomain inverted layers 3 for 1060 nm, oscillation was achieved at 1060nm, whereby green laser light (wavelength: 530 nm) was obtained as theharmonic wave P2. Moreover, by changing the coating on the totalreflection mirrors 22 and 23 for reflection of the 1300 nm band, andchanging the pitch of the domain inverted layers 3 for 1300 nm,oscillation was achieved at 1300 nm, whereby red laser light(wavelength: 650 nm) was obtained as the harmonic wave P2. With thisconfiguration, primary color laser light, i.e., blue, green and redlight, can be easily obtained. Next, FIG. 25 illustrates anotherconfiguration where the solid state laser crystal and the opticalwavelength conversion element are separated from each other. Nd:YVO₄ asthe solid state laser crystal 21 was attached to the output side of thefiber. The domain inverted structures are periodically formed in theoptical wavelength conversion element 25 of the LiTaO₃ substrate 1. Bluelaser light of 2 W was stably obtained also by the laser light source ofthis configuration.

Still another example of the present invention will be describedreferring to the figures. FIG. 26 illustrates a configuration of a laserlight source according to the present example. The pumped light P1 aemitted from the semiconductor laser 20 having a wavelength of 806 nm isconverted to the fundamental wave P1 by the solid state laser crystal21, is incident upon the fiber 40, and propagates through the fiber 40.The fiber 40 is a single mode fiber. The fundamental wave P1 emittedfrom the fiber 40 enters the optical wavelength conversion element 25.In this example, the optical waveguide 2 which is produced in the LiTaO₃substrate 1 utilizing proton exchange is used as the optical wavelengthconversion element 25 with periodic domain inverted structures. In thefigure, reference numeral 1 denotes an LiTaO₃ substrate of a Z plate; 2denotes a formed optical waveguide; 3 denotes a domain inverted layer;10 denotes an incident portion for the fundamental wave P1; and 12denotes an emitting portion for the harmonic wave P2. The fundamentalwave P1 which has entered the optical waveguide 2 is converted to theharmonic wave P2 by the domain inverted layers 3. Thus, the harmonicwave P2 having the power increased in the optical waveguide 2 isradiated out through the emitting portion 12. The radiated harmonic waveP2 is collimated by the lens 32.

Moreover, the electrode 14 is formed on the element via the protectivefilm 13. The harmonic wave P2 of 10 W was obtained for the pumped lightP1 a of 30 W. The blue laser light was modulated at 30 MHz by applying amodulation signal to the electrode 14 formed on the optical wavelengthconversion element 25. The conversion section of the laser light sourceaccording to this example has a length of 10 mm, and it can be made verycompact by designing it so that the fundamental wave P1 propagatesthrough a fiber. Moreover, it is possible to prevent temperaturevariation by remotely disposing the optical wavelength conversionelement away from the semiconductor laser.

FIG. 26 illustrates an example where a solid state laser crystal is notused.

A semiconductor laser of 980 nm, having an output of 10 W, is used. Thisis coupled to the optical wavelength conversion element 25 through thefiber 40 so as to perform direct conversion. An output of 2 W wasobtained for a wavelength of 490 nm.

Next, referring to FIG. 27, a laser projection apparatus of the presentinvention will be described. Light sources of three colors, i.e., theblue laser light source, the green laser light source and the red laserlight source of Example 5 were used as light sources. Reference numeral45 denotes a blue laser light source in the 473 nm wavelength band.Reference numeral 46 denotes a green laser light source in the 530 nmwavelength band; and 47 denotes a red laser light source in the 650 nmwavelength band. A modulation electrode is attached to each of theoptical wavelength conversion elements. Each light source output ismodulated by inputting a modulation signal to the modulation electrode.The green laser light is combined with the blue laser light by the adichroic mirror 61. Moreover, the red laser light is combined with theother two colors by a dichroic mirror 62. Reference numeral 56 denotes avertical deflector, and 57 denotes a horizontal deflector, both of whichuse a rotating polygon mirror. A brightness of 2000 cd/m², a contrastratio of 100:1, a horizontal resolution of 1000 TV and a verticalresolution of 1000 TV were obtained for a screen size of 2 m×1 m using ascreen 70 having a gain of 3. Thus, the laser projection apparatus ofthe present invention is significantly effective in providing a highbrightness, a high resolution and an extremely low power consumption.

Although an optical wavelength conversion element of the domain invertedtype is used in the present example, it is not limited thereto.Moreover, when a laser light source which is directly oscillated by asemiconductor laser is used for red color, the cost can be furtherreduced. Alternatively, the semiconductor laser direct oscillation typemay be used as the blue or green laser. The combination thereof may befreely determined.

Moreover, in the present example, the following features have beendevised for safety. The laser is designed to turn off automatically whenthe laser light scanning is terminated. Infrared laser light beingsub-semiconductor laser with a weak output is scanning around theprojected laser light, and it is designed so that the laser light isautomatically turned off when an object contacts the light. An infraredsemiconductor laser has a feature of a low cost and a long life.

Next, these will be described referring to FIG. 28. The three laserlight beams of the primary colors are scanned by the deflector within adisplay area 71 of the screen 70. The laser light passes over sensors Aand B located on the periphery of the display area 71. Output signalsfrom the sensors A and B are always being monitored. On the other hand,the laser light from the infrared laser light source of an infraredsemiconductor laser is always scanned by a deflector 58 along theperiphery of the screen 70. The reflected light enters a sensor C. Thatis, it is designed so that light reflected at any position along theperiphery enters the sensor C.

Next, control will be described referring to FIG. 29. In FIG. 29, when asignal from either one of the sensors A and B does not enter a controlcircuit for a certain period of time, the main power of the laser lightsource is turned off so that the blue, red and green laser light sourcesare turned off. That is, by terminating the scanning, it is possible toprevent a certain position from being irradiated with the laser light ina concentrated manner. Moreover, if the signal from the sensor C isdiscontinued even for a moment, the power of the laser light source isturned off by the control circuit. Thus, it is safe since a human, etc.,never touches the short wavelength laser light having a high output. Thesafety of this laser projection apparatus is maintained as describedabove.

Although the power of the laser light source is turned off in theexample, it is also applicable to block the optical path of the laser.Moreover, generation of short wavelength laser light may be terminatedby shifting the phase-matched wavelength of the optical wavelengthconversion element using a voltage or the like, or by varying theoscillation wavelength of the semiconductor laser as a fundamental wavelight source. This method allows for considerable reduction of theperiod of time required for a restart.

Next, referring to FIG. 30, an example of a three-dimensional laserprojection apparatus of the present invention will be described.

This is an apparatus which provides a stereoscopic view for a viewer.FIG. 30 illustrates a configuration of the laser projection apparatusaccording to the present example. As shown in FIG. 30, by inserting aprism type optical path convertor 66 for three color laser light, thelaser light is split into two directions. The split laser light beamsare reflected by respective mirrors 64 and 65 and modulated bymodulators 5 a and 5 b, so as to be incident upon the screen 70. Animage viewed from the right direction and an image viewed from the leftdirection are respectively superimposed by the modulators 5 a and 5 b,and the light beams are incident upon the screen 70 from differentdirections, thereby being viewed stereoscopically. Moreover, an opticalpath 1 and an optical path 2 are switched with each other at regularintervals so that a human feels as if different images are coming fromtwo directions, thereby making the stereoscopical image even clearer. Asin this example, a stereoscopic image can be easily viewed withoutstereoscopic glasses.

A stereoscopic view can also be realized by splitting light in two by ahalf mirror, or the like. Moreover, although a single light source issplit in the above example, two laser light sources of the same colormay be used to irradiate the screen from different directions. In thiscase, only half the output is required for each light source.

Next, still another example of a laser projection apparatus of thepresent invention will be described.

FIG. 31 illustrates a configuration of the laser projection apparatusaccording to the present example. An ultraviolet laser light sourcebased on the optical wavelength conversion element is used as a lightsource. By irradiating the screen 70 on which a fluorescent substance isapplied with the light, RGB light, i.e., red, green and blue light, isemitted. In the configuration of the laser light source, 650 nm redlaser light directly oscillated by a semiconductor laser was made tohave half the wavelength, i.e., 325 nm, by an optical wavelengthconversion element of LiTaO₃. The optical wavelength conversion elementis of the bulk type in which the domain inverted structures are formed.Reference numeral 48 denotes the laser light source. Herein, anultraviolet modulation signal is obtained by directly modulating redsemiconductor laser. The modulated ultraviolet laser light enters adeflector. Reference numeral 56 denotes a vertical deflector, and 57denotes a horizontal deflector, both of which use a rotating polygonmirror. The screen 70 has fluorescent substances applied thereonrespectively for generating red, green and blue light, and thusgenerates fluorescent light. A brightness of 300 cd/m², a contrast ratioof 100:1 and a horizontal resolution of 600 TV were obtained for ascreen size of 1 m×0.5 m. As in this example, it is possible to generatethe primary colors, i.e., red, green and blue, with a single laser lightsource, whereby it is possible to realize smaller size and lower cost.It is also favorable that the dichroic mirror for combining waves can beeliminated.

Next, referring to FIG. 32, a laser projection apparatus of the presentinvention will be described. As shown in FIG. 32, the blue laser lightsource 45 based on the optical wavelength conversion element is used asa light source. Laser light emitted from the laser light source 45 iscollimated by the lens 30. A liquid crystal light bulb 68 is inserted inthe collimated laser light. Light is spatially modulated by applying asignal to the liquid crystal light bulb 68, enlarged by the lens 31 andprojected onto the screen so that an image can be viewed. This can bedone in multi-colors by using laser light sources of the primary colors.

As compared to the conventional technique, the efficiency wasconsiderably improved and the power consumption was reduced. Moreover,it is also advantageous in that the amount of heat generated is small.

Next, referring to FIG. 32, a laser projection apparatus of the presentinvention will be described. The configuration in appearance is the sameas that in the example of the laser projection apparatus illustrated inFIG. 20. As a light source, the blue laser light source of FIG. 23 isused, and the laser light source used herein is RF superimposed.Moreover, the blue light is modulated by an input of a modulation signalin addition to the RF superimposition. The modulated blue laser light isincident upon a deflector. A brightness of 200 cd/m² was obtained for ascreen size of 2 m×1 m using a screen having a gain of 2. A specklenoise to be generated due to laser light interference was not observedon the screen. This results from the reduction of coherency of the laserlight by the RF superimposition, and the RF superimpositionsignificantly contributes to countermeasures for the speckle noise.While the laser light source configuration of FIG. 23 is used in thepresent example, the RF superimposition is effective for a laserprojection apparatus using a laser light source based on the directwavelength conversion of semiconductor laser. Moreover, a speckle noisecan also be prevented in the case where red, green or blue laser lightis generated directly by semiconductor laser light. It is needless tosay that it is also effective for a color laser projection apparatus.

Although LiNbO₃ and LiTaO₃ are used as a nonlinear optical crystal inthe above-described example, it is also applicable to use aferroelectric substance such as KNbO₃ or KTP, an organic material suchas MNA, and other materials obtained by doping a rare earth element intothese materials. Moreover, as a rare earth element, Er or Tl is alsoprospective in addition to Nd which is used in the examples.Furthermore, although YAG is used as a solid state laser crystal, othercrystals such as YLF or YVO₄ are also effective. LiSAF and LiCAF arealso effective solid state laser crystals.

Next, referring to FIG. 33, an example where the laser light source ofthe present invention is applied to an optical disk apparatus will bedescribed.

The optical disk apparatus has the optical wavelength conversion element25, which includes the domain inverted structures, within an opticalpickup 104, whereby the laser light emitted from the semiconductor laser20 is passed to the optical wavelength conversion element 25 within theoptical pickup 104 via the fiber 40.

In addition to the optical wavelength conversion element 25, the opticalpickup 104 includes: a collimator lens 32 for converting a harmonic waveemitted from the optical wavelength conversion element 25 to acollimated light; a polarization beam splitter 105 for transmitting thecollimated light to the optical disk; a collective lens 106 forcollecting the light onto the optical disk; and a detector 103 fordetecting reflected light from the optical disk. The polarization beamsplitter 105 selectively reflects the reflected light from the opticaldisk so as to pass it to the detector 103.

While the optical pickup 104 is driven by an actuator, the semiconductorlaser 20 is fixed in the optical disk apparatus. The optical pickup 104can reliably receive, by the flexible optical fiber, laser light fromthe semiconductor laser 20 fixed in the optical disk apparatus.

Next, the operation will be described.

Light (pumped light) emitted from the semiconductor laser 20 isconverted to the fundamental wave P1 by the solid state laser 21, andradiated onto the optical wavelength conversion element 25. The opticalwavelength conversion element 25 has a configuration similar to that inthe above-described example and converts the fundamental wave P1 to theharmonic wave P2. The harmonic wave P2 is collimated by the collimatorlens 32, passes through the polarization beam splitter 105, and is thencollected onto the optical disk medium 102 via the collective lens 106.The reflected light from the optical disk medium 102 returns by the sameoptical path again, is reflected by the polarization beam splitter 105,and is detected by the detector 103.

Thus, a signal can be recorded on the optical disk medium, or a signalrecorded thereon can be reproduced.

A quarter-wave plate 108 is inserted between the polarization beamsplitter 105 and the collective lens 106 so as to rotate a polarizationdirection of a harmonic wave by 90 degrees on its way out and on its wayback in.

When a semiconductor laser having an output of 1 W was used as thesemiconductor laser 20, a harmonic wave P2 of 200 mW was obtained. Thewavelength of light emitted from the solid state laser 21 is 947 nm, andthe wavelength of the harmonic wave is 473 nm.

By using high power laser light having an output of 200 mW, it ispossible to perform a recording operation at a speed 10 times fasterthan the recording speed achieved by an optical disk apparatus usingconventional 20 mW output light. The transfer rate was 60 Mbps.

Moreover, the semiconductor laser 20 which generates heat duringoperation is fixed in a housing of the optical disk apparatus, and isremote from the optical pickup. Thus, as a result of removal of thesemiconductor laser from the optical pickup, it is no longer necessaryto provide a special heat release structure for a semiconductor laser.It is thus possible to compose a subminiature and light weight opticalpickup. As a result, the optical pickup can be driven by an actuator ata high speed, whereby a fast recording operation at a high transfer ratecan be achieved.

Although the solid state laser is located on the side of thesemiconductor laser in the present example, it may also be located onthe side of the optical wavelength conversion element. Moreover, it isapplicable to convert light from the semiconductor laser as afundamental wave directly to a harmonic wave without using a solid statelaser.

The internal structure of the optical pickup 104 is not limited to thatof the present example. For example, by using a polarization separatinghologram, it is possible to eliminate a lens and a polarization beamsplitter. Thus, the optical pickup can be made further smaller.

INDUSTRIAL APPLICABILITY

As described above, in the optical wavelength conversion element of thepresent invention, after an optical element is produced in anLiNb_(x)Ta_(1-x)O₃ (0≦X≦1) substrate, low-temperature annealing isperformed so as to repress an increase in refractive index generatedduring a heat treatment such as high-temperature annealing, and then, astable proton exchange layer is formed, whereby a stable optical elementcan be formed. Particularly, the present invention is indispensable forputting into practical use an optical wavelength conversion elementwhose phase-matched wavelength varies with refractive index variation.

Moreover, the 2-stage annealing with two different temperatures iseffective as the low-temperature annealing since it enables a stableproton exchange layer such that there is completely no temporalvariation to be restored quickly. It is further effective since thestrain can be greatly mitigated and a stable proton exchange layer canbe formed by performing second annealing at a temperature lower thanthat in first annealing by 200° C. It is further effective since thetemporal variation is 0.5 nm or less if the low-temperature annealing isperformed for at least 1 hour at a temperature of 120° C. or lower, andit is particularly effective if the temperature is 90° C. or less,whereby the phase-matched variation is small. If the temperature is 50°C. or lower, there will be a problem of an extremely long annealingtime. Therefore, the annealing needs to be performed at a temperaturethereabove.

Moreover, with the laser light source of the present invention, it ispossible to stabilize the oscillation wavelength of the semiconductorlaser and to increase the fundamental wave output by inserting asemiconductor laser amplifier between a distributed feedback typesemiconductor laser and an optical wavelength conversion element, whileit is also possible to stably obtain the maximum harmonic wave output byusing the highly efficient optical wavelength conversion element havingdomain inverted layer structures.

Furthermore, with the laser light source of the present invention, theoptical wavelength conversion element section can be made very compactby designing it so that pumped light or a fundamental wave propagatesthrough a fiber. Furthermore, it is possible to remotely dispose theoptical wavelength conversion element away from the heat generated bythe semiconductor laser and thus to prevent temperature variation,whereby a high output semiconductor laser can be used.

Moreover, if periodic domain inverted structures are used as the opticalwavelength conversion element, in addition to a significantly improvedconversion efficiency, modulation can be easily effected by applying alow voltage, thereby presenting an industrial advantage. Thus, amodulator can be integrated, whereby it is possible to achieve smallersize, lighter weight and lower cost. Furthermore, by employing anoptical waveguide with a weak confinement as the optical wavelengthconversion element, the concentration of a harmonic wave becomes small,whereby the optical damage is considerably improved. This is because, a100 times larger area with respect to that in a conventional techniquecan tolerate a 100 times greater optical damage. Furthermore, with thelaser light source of the present invention, it is possible to use ahigh output semiconductor laser of a multi-stripe or wide-stripe type byconverting pumped light to a fundamental wave using a solid state lasercrystal, whereby it is possible to obtain a high output harmonic wave.

Because of these factors, it is possible, for example, to obtain a totalconversion efficiency of 20% by amplifying the electro-opticalconversion efficiency of the semiconductor laser of 30% with theconversion efficiency of an optical wavelength conversion element of70%. Moreover, by RF superimposing a semiconductor laser in the laserlight source of the present invention, the conversion efficiency isimproved by 5-fold, for example, as compared to the case when the RFsuperimposition is not performed.

Furthermore, with the laser projection apparatus of the presentinvention, since it is based on a semiconductor laser, it is possible toachieve considerably smaller size, lighter weight and lower cost.Moreover, it is possible to simultaneously achieve a smaller size, alighter weight and a lower cost of the apparatus by using a high outputlaser light source based on a semiconductor laser and an opticalwavelength conversion element. Furthermore, the power consumption canalso be extremely low. One of the factors therefor is that the apparatusdoes not separately has a modulator for laser light, which, instead, isintegrated with the optical wavelength conversion element. Furthermore,as compared to a conventional technique, the resolution is considerablyimproved. For example, as compared to a configuration using a gas laser,considerable improvements are achieved such as a one thousandth weight,a one-thousandth volume and a one-hundredth power consumption. The smallsize and the low power consumption of the employed laser light sourceand the integration thereof with the optical modulator greatlycontribute to these achievements. That is, this results from that theconfiguration using a semiconductor laser and an optical wavelengthconversion element can be subminiaturized while the efficiency ofconversion from an electrical power is higher than that of a gas laserby about two orders of magnitude. Particularly, it is significantlyeffective to use an element having periodic domain inverted structuresas an optical wavelength conversion element, since this makes itpossible to improve efficiency and realize integration of the opticalmodulator driven with a low voltage.

Furthermore, it is possible to generate the primary colors by allowingfluorescent substances to be irradiated with an ultraviolet laser lightsource, and thus to achieve an even smaller size and lower cost, therebypresenting a significant industrial advantage. Thus, it is possible togenerate the primary colors, i.e., red, green and blue, with a singlelaser light source. It is also favorable that the dichroic mirror forcombining waves can be eliminated.

Furthermore, when scanning is terminated, the laser projection apparatusof the present invention prevents a certain position from beingirradiated with laser light in a concentrated manner, thereby presentinga laser light termination or cutting function. Moreover, if the signalfrom a sensor is interrupted even for a moment, the power of the laserlight source is turned off by a control circuit. Thus, it is safe sincea human, etc., never touches the short wavelength laser light having ahigh output. The safety of this laser projection apparatus is maintainedas described above.

Furthermore, the RF superimposition is effective for a laser projectionapparatus using a laser light source based on direct wavelengthconversion of a semiconductor laser. This is because a speckle noise canbe prevented from being generated, whereby a clear image can bereproduced. Moreover, a speckle noise can also be prevented in the casewhere red, green or blue laser light is generated directly bysemiconductor laser light.

1. A laser device, comprising: at least one laser light source includinga semiconductor laser; a first optical system for irradiating a spatialmodulation element with laser light emitted from the laser light source;and a second optical system for irradiating a screen with the lightemitted from the spatial modulation element, wherein the laser lightsource further includes: bulk type optical wavelength conversion elementin which periodic domain inverted structures are formed, and a singlemode fiber for conveying laser light from the semiconductor laser to theoptical wavelength conversion element, wherein the single mode fiber isconfigured to prevent a variation in temperature of the opticalwavelength conversion element caused by a heat generated from thesemiconductor laser.
 2. A laser device according to claim 1, wherein thespatial modulation element is a liquid crystal cell.
 3. A laser deviceaccording to claim 1, wherein the laser light source further includes anoptical waveguide for guiding the laser light from the semiconductorlaser.
 4. A laser device according to claim 1, wherein the semiconductorlaser is wavelength-locked.
 5. A laser device according to claim 4,wherein wavelength-locking is performed using a grating or a filter.